Seeing^ Hearing, and Smelling 5 i 



the Scallops (Pedcn), have a considerable develop- 

 ment of eyes upon the fringes of the mantle, and it 

 has been reasonably surmised that these eyes enable 

 them to guide themselves in their vigorous zigzag 

 flights through the waters. But it may be regarded 

 as tolerably certain that these eye-substitutes, or 

 pigment spots, that are crowded upon certain parts 

 of the mantle in many bivalves are merely sensitive 

 to varying qualities of light and are useless for 

 observation. The casting of a shadow upon such 

 species affects these pigment spots, and from them 

 the intelligence is conveyed along the nerves to the 

 various ganglia, with the result that as a precaution- 

 ary measure, the foot and siphons are withdrawn 

 and the valves are shut with a snap. The Land- 

 snails {Helix) being crepuscular or nocturnal in their 

 habits can see farther in twilight than at midday, 

 but their visual powers are very limited at the 

 best, and extend only to a distance of about six 

 millimetres. The Round - mouthed Snail (Cyclo- 

 stoma elegans), however, whose eyes are not 

 mounted on tentacles, can see to a distance of about 

 a foot. 



The sense of hearing has not been very extensively 

 investigated, at least little but negative results have 

 been obtained from experiments made with a view 

 to testing its extent. One experiment of the kind 

 made by Mr. W. Bateson in the Plymouth Laboratory 

 of the Marine Biological Association, a few years 

 ago, showed that a Saddle-oyster (Anomiia) sus- 

 pended in the water to be free from any chance 

 of jarring of the solidly- framed tank, closed its 

 valves on Mr. Bateson making a creaking sound 



